There are many ways of driving motion transmission cables, whether or not wound in a loop but wherein the motor speed should almost always be geared down to increase the force transmitted by the cable. One system encountered includes two pulleys of different diameters on which the cable is wound, possibly making several revolutions around the pulley of smaller diameter in order to increase grip. The small pulley is the driver and communicates a smaller rotation but a more substantial torque to the large pulley. A drawback of this system is that it has a large space requirement, the two pulleys having significant minimum diameters, that slipping is difficult to eliminate completely and that the cable portion on the small pulley is subject to a significant angular speed which increases its fatigue and may therefore cause it to wear quickly.
Another system includes gear reductions, but these mechanical means include substantial play which makes them inaccurate and their space requirement is still significant. In other designs, cable movements are controlled by means of a block gearing down the motion between a drive cable and a driven cable, but blocks are complex pieces of equipment and the reliability of the whole is not guaranteed.
One final group of motion to cable transmissions, to which the invention is related, includes screw and nut systems one of the elements of which is driven by the motor and the other is attached to the cable. These systems achieve excellent speed reduction and good nominal resistance, but a significant problem emerges in that the components have to be guided and that possible alignment defects have to be taken into account between the screw, the nut and the cable.
This latter phenomenon causes significant friction between the screw and the nut and bending moments on the screw, which compromise the yield and lifespan that might have been expected. Mechanisms responsible for guiding the screw and the nut while keeping them in alignment, and which might have overcome these defects, are too cumbersome or increase friction still further. It is for this reason that the proposal has been made (in French patent 2 782 467) to mount the nut on a frame using a cardan joint which allows it to follow the tilting movements of the screw, to which the cable or another connection component is attached, but the cardan joint must surround the nut in such a way that its axes of rotation converge towards the centre of the nut, which gives too voluminous an assembly for many applications. Another cardan joint is moreover provided between the screw and the connection component to which it is attached.
A different means is therefore used to improve the alignment between the transmission component such as the cable, the screw to which this component is attached and the nut, or at least to attenuate the effects of alignment defects.